Completing Mirroring Halves
Go to the modifiers tab, make sure you are in Object mode.  Currently you can change one side of the object, and it will automatically update the other side.  Click the "Apply" button, and the mirror modifier will be applied to your object.



Now that the modifier is applied, I went back into Edit mode, and dragged a vertex around, and you can see that it does not change the other side of the object.



Now you can continue modeling!